Foraminiferida

(invertebrate zoology) An order of dominantly marine protozoans in the subclass Granuloreticulosia having a secreted or agglutinated shell enclosing the ameboid body.

An order of Granuloreticulosia in the class Rhizopodea. Foraminiferans are dominantly marine protozoans, with a secreted or agglutinated shell, or test, enclosing the continually changing ameboid body (see illustration) that characterizes this and other orders of the superclass Sarcodina. Their unique combination of long geologic history, ubiquitous geographic distribution, and exceptional diversity of test composition, form, and structure make the foraminiferans the most useful of all marine fossils for stratigraphic correlation, geologic age dating of sediments, and paleoecologic interpretation. Their tests accumulated in great numbers and are recoverable from small quantities of sediment, rock outcroppings, well cores or cuttings, or ocean dredging and submarine coring.

elongate test. (b) Uvigerina, elongate triserial test. (c) Siphogenerinoides, reduced early biserial stage. Superfamily Spirillinacea; Patellina: (d) spiral view of low conical test; (e) umbilical view. (R. B. MacAdam, Chevron Oil Field Research Co.)">
Scanning electron micrographs of foraminiferans of suborder Rotaliina. Superfamily Buliminacea: (a) Eouvigerina, elongate test. (b) Uvigerina, elongate triserial test. (c) Siphogenerinoides, reduced early biserial stage. Superfamily Spirillinacea; Patellina: (d) spiral view of low conical test; (e) umbilical view. (R. B. MacAdam, Chevron Oil Field Research Co.)

Pseudopodia from the ameboid body form a delicate anastomosing network of 2–10 times the test diameter. The pseudopodial net may arise from the apertural region of the test alone in those with tectinous, porcelaneous, and agglutinated walls, or may radiate in all directions through many tiny perforations of the hyaline test wall. The pseudopodia variously serve in capture, ingestion, and digestion of food, in test and temporary cyst construction, for anchorage, and for locomotion. The characteristic granular streaming of the continuously moving cytoplasm differentiates the Foraminiferida from the other Sarcodina orders.

The foraminiferan constructs its own test. Growth of the individual may cease after test construction, or the test may enlarge by continued growth in one or more directions (tubular or branching tests) or by periodic formation of separate but always interconnected chambers. Growth may be either continuous or periodic and may be in a straight line, a planispiral or trochospiral coil, a cycle, or a zigzag. The latter (zigzag) is expressed by chambered forms in biseriality. Variations and combinations of these growth patterns occur repeatedly in different lineages (isomorphism).

As is true of most protists with skeletons or tests, systematic differentiation and classification of foraminiferans is based on test composition, microstructure, and gross morphology. Information currently available concerning cytoplasmic characters, life cycles, and so on has shown good agreement with this classification, although the function and origin of many shell characters believed to be of systematic importance (canal systems, pores, septal doubling, and apertural tooth plates) are yet undetermined. There are 11 suborders.

Foraminiferans have shown a remarkable diversification and rapid evolutionary development over their 500,000,000-year known history. Some with uncomplicated tests have been reported to have a long geologic range, but most with diagnostic shell structure and morphology had relatively short histories, each successively replaced by others. The oldest-known and presumably primitive tests are the morphologically simple tectinous and agglutinated one of the Cambrian and Ordovician, globular, tubular, branching, or irregular form.

Most foraminiferans are benthic, living upon the sea floor, within the upper few centimeters of ooze, or upon benthic algae or other organisms. They occur from the intertidal zone to oceanic depths, in brackish, normal marine, or hypersaline waters, and from the tropics to the poles. Some modern Lagynacea live in fresh water, but none are known as fossils. Assemblages vary widely in response to local conditions, with the greatest diversity occurring in warm, shallow water. A smaller number, the Globigerinina, are planktonic, living at various depths in the water column from the surface to the bottom, being most numerous between 18 and 90 ft (6 and 30 m). Vertical migration may be diurnal and may occur during ontogenetic development. The preferred depth range of a species may vary geographically in response to temperature differences or to changes in water density. See also Granuloreticulosia; Rhizopodea; Sarcodina.